The batteries should come with a vent line that goes on the bank of openings in the top of the battery. If not, there should be a standard round hole that you can push your existing nipple into. If you do not have a vent line, a trip to the auto parts store should yield a hose and a nipple that fits into your existing lines. You can even just make this from scratch and find a grommet to the outside.
4 yrs old this month (Battery and Spark Plugs questions)
HTH,
G